Try a full reset, power down and (count to 10) power back up... I found that these do not like hidden SSID and will fail to connect, also if the AP is restarted, updated, it needs to be powered off and on again.
It could also be that the battery is essentially dead and must be line powered in order to connect. Note sure how or if the battery can be replaced.
Other than that it is prob. become a light weight brick... these were some of the first smart PM2.5 sensors and are great in the APP ... when working.. ^_^
GL